Differential circuits have been employed in wireless cellular communications handsets and other wireless systems for many years. The principal benefits from using differential circuits as opposed to single-ended circuits are lower noise and lower susceptibility to interference. Despite the benefits of differential circuits, some of the components used in a modern wireless communications systems remain single-ended. For example, single-ended antennae are more common than differential antennae. In cases where wireless communications systems include single-ended and differential components, it is necessary to include devices which convert the single-ended signals which are incident on and emitted from the single-ended components to differential signals which are incident on and emitted from the differential components. Conversely, devices which convert the differential signals which are incident on and emitted from the differential components to single-ended signals which are incident on and emitted from the single-ended components are also required.
Such devices are often referred to as baluns. Figures of merit for describing the electrical characteristics of a balun which converts a single-ended signal to a differential signal are the single-ended to differential mode response, the single-ended to common mode response, the amplitude balance, and the phase balance. Figures of merit for describing the electrical characteristics of a balun which converts a differential signal to a single-ended signal are the differential mode to single-ended response, the common mode to single-ended response, and the amplitude and phase balance.
A balun can be implemented by a number of discrete components. Balun topologies employing discrete components are taught in U.S. Pat. No. 5,949,299 (Harada) and U.S. Pat. No. 6,396,362 (Mourant et al). Baluns can also be implemented using distributed components, normally employing pairs of serially connected quarter-wavelength coupled lines. A popular form of the distributed balun is often referred to as a Marchand balun. A variation of the Marchand balun is taught in U.S. patent US06292070 (Merrill) and is commonly referred to as a backwards-wave balun. Distributed baluns such as the Marchand balun and the backwards-wave balun typically offer excellent performance over a wide bandwidth.
FIG. 1 shows the structure of a conventional Marchand balun, the structure has a first pair of transmission lines 10A, 10B, a second pair of transmission lines 11A, 11B, a differential port 12 and a single-ended port 13. The length of each transmission line is substantially equal to one quarter of the wavelength at the centre of the operating frequency band of the balun. During use, electromagnetic coupling occurs between the respective transmission lines in each pair 10A, 10B and 11A, 11B.
The structure of a backwards-wave balun is depicted in FIG. 2, the structure has a first pair of transmission lines 20A, 20B, a second pair of transmission lines 21A, 21B, a differential port 22 and a single-ended port 23. The length of each transmission line is substantially equal to one quarter of the wavelength at the centre of the operating frequency band of the balun. During use, electromagnetic coupling occurs between the respective transmission lines in each pair 20A, 20B and 21A, 21B.
FIG. 3 shows a size-reduced Marchand balun as described in Gavela I., Falagan M. A., Fluhr H.; “A Small Size LTCC Balun for Wireless Applications”; Proceedings of the European Microwave Conference 2004; pp 373-376. The structure of this balun is similar to that of FIG. 1 and includes a first pair of transmission lines 30A, 30B, a second pair of transmission lines 31A, 31B, a differential port 32 and a single-ended port 33. In addition, two shunt capacitors 34A and 34B are provided, as shown, with the effect that the required length of each of transmission lines 30A, 30B and 31A, 31B is less than one quarter of the wavelength at the centre of the operating frequency band of the balun.
FIG. 4 shows another size-reduced balun as taught in U.S. Pat. No. 6,801,101 (Hiroshima et al); the balun includes a first pair of transmission lines 40A, 40B, a second pair of transmission lines 41A, 41B, a differential port 42 and a single-ended port 43. The balun additionally includes shunt capacitors 44A, 44B, 45A, 45B and 46, as shown, an effect of which is that the required length of each of the transmission lines is less than one quarter of the wavelength at the centre of the operating frequency band of the balun.
Where an antenna having a single-ended input/output (I/O) port is required to be connected to a differential I/O port of a transceiver, a balun is required. However, connection from the single-ended I/O port of the antenna to the differential I/O port of the transceiver is not always readily achievable using conventional baluns. For example, FIG. 5 shows a chip antenna 50 as taught in U.S. Pat. No. 7,042,402 (Modro). The chip antenna 50 is typically mounted on a PCB or substrate 57 so that the main radiating section is at a raised elevation relative to the surface of the substrate 57. Consequently, the co-planar single-ended-port of the antenna, comprising signal-carrying terminal 52 and ground terminals 53B and 53B, are also at a raised elevation relative to the surface of the substrate 57. On the other hand, the terminals (not shown) of the differential port of a transceiver (not shown), or other transmitter or receiver circuitry, to which the antenna is to be connected via a balun are normally located on the surface of the substrate 57.
Unfortunately, none of the conventional balun circuits shown in FIGS. 1, 3 or 4 can be easily arranged to provide a balun with signal-carrying terminals appropriately located on upper and lower faces to allow the antenna 50 to be coupled to a transceiver because, in each case, the single ended I/O port is located at one end of the balun structure.
The I/O ports of the backwards wave balun of FIG. 2 are suitably positioned so that a backwards wave balun could be arranged to provide signal-carrying terminals located as required on the upper and lower faces of the balun. However, the requirement for three direct connections to electrical ground on the single-ended side of the backwards-wave balun of FIG. 2 represents a significant design challenge at radio frequencies (RF), because the required elevation of the signal-carrying terminal of the single-ended port of the balun, and the required lower location of the terminals of its differential port dictate that coupled line sections 20A and 21A should be located towards the lower section of the balun and coupled line sections 20B and 21B should be located near the upper section of the balun. Hence, the ground connections would necessarily be made using long metal filled via holes inside the balun, but there is an inevitable parasitic inductance associated with such via holes.